1. Field of the Invention
This invention relates to methods making stents from bioabsorbable polymers.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices that function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
Stents are typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts, that may be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. This scaffolding gets its name because it physically holds open and, if desired, expands the wall of the passageway, or lumen. Typically, stents are capable of being compressed, or crimped, onto a catheter so that they can be delivered to and deployed at a treatment site. Delivery includes inserting the stent through small lumens, such as blood vessels, using a catheter and transporting it to the treatment site. Deployment includes expanding the stent to a larger diameter once it is at the desired location. Mechanical intervention with stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis remains a significant problem. When restenosis does occur in the stented segment, the treatment of it can be challenging, as clinical options are more limited than for those lesions that were treated solely with a balloon.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy uses medicated stents to locally administer a drug. A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes a drug. Polymeric scaffolding may also serve as a carrier of a drug.
It may be desirable for a stent to be biodegradable. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers should be configured to completely erode only after the clinical need for them has ended.
One of the challenges of making medical devices out of polymers is that the properties of a polymer can change both during processing and after processing. These properties include mechanical properties such as strength and toughness as well as bioresorption kinetics. The processing steps in a fabrication process of a stent may be designed to maintain or instill in the stent particular ranges of the strength, toughness, and bioresorption, that are crucial for treatment with the stent. In some cases, properties of the polymer can change during additional processing operations and/or as a function of time during storage. Therefore, methods are needed that reduce, or eliminate undesirable changes in properties, and/or ameliorate their impact.